If division occurs in _two planes only_, there may result the above groupings and several others in addition. The four cocci which result from a single division may remain together, giving the _tetracoccus_ or _tetrad_ grouping. Very rarely all the cocci divide evenly and the result is a regular _rectangular flat mass_ of cells, the total number of which is a multiple of four. The term merismopedia (from a genus of algæ which grows the same way) is applied to such a grouping. If the cells within a group after a few divisions do not reproduce so rapidly (lack of food), as usually happens, the number of cells becomes uneven or at least not necessarily a multiple of four and the resultant _flat mass_ has an _irregular_, _uneven outline_. This grouping is termed _staphylococcus_ (σταφυλος = a bunch of grapes) (Fig. 47). It is the most common grouping among the cocci. When division occurs in all three planes, there is in addition to all the groupings possible to one- and two-plane division a third grouping in which the cells are in _solid packets_, _multiples of eight_. The name _sarcina_ is applied to this growth form (Fig. 48). The individual cells in a sarcina packet never show the typical coccus form so long as they remain together, but are always flattened on two or more sides. The above descriptions indicate how the method of division may be determined. If in examining a preparation the _sarcina_ grouping appears, that shows _three-plane division_. If there are no sarcina, but _tetrads_ or _staphylococci_ (rarely merismopedia), then the division is in _two planes_. If none of the foregoing is observed but only _diplo-_ or _streptococci_, these indicate _one-plane division_ only. Cocci show their _characteristic_ groupings only when grown in a liquid medium, and such should always be used before deciding on the plane of division. [Illustration: FIG. 47.--Staphylococcus grouping. The large flat masses are staphylococcus grouping. Diplococcus grouping, tetrads and short streptococci are also evident.] [Illustration: FIG. 48.--Sarcina grouping.] As the above description shows, these terms which are properly adjectives describing the cell grouping, are quite generally used as nouns. Thus the terms a diplococcus, a tetrad, a streptococcus, etc., are common, meaning a bacterium of the cell form and cell grouping indicated. CELL FORM. CELL GROUPING. coccus-- {diplococcus--in 2’s. round or spherical. {streptoccus--in chains. {tetracoccus, tetrads--in 4’s. {staphylococcus--irregular flat masses. {sarcina--regular, solid packets, multiples of 8. bacillus-- {diplobacillus--in 2’s. rod-shaped {streptobacillus--in chains. or cylindrical. spirillum-- {diplospirillum--in 2’s, little used. spiral-shaped. {streptospirillum--in chains, little used. CHAPTER V. CLASSIFICATION.